EMI - Electro magnetic Interference

Action: Undesired signals couple into signals via four coupling factors;

1. Magnetic (inductive) coupling
This would be electrical interference caused by magnets or electromagnets.

Only seen that once - when an entheusiastic employee neatly coiled up the excess length of speed sensor and load cell cabling and zip tied it inside the eddy current brake housing, next to the brake. Whne enough load came on, the speed signal was corrupted.

2. Capacitive (high speed voltage changes (dV/dt)) coupling,

4. Radiative (Rf) Coupling.

This is the most likely to occasionally appear in a dyno room.

It can be caused by a spark jumping a gap, as in a spark plug gap - OR - an arcing spark to ground from a bad ignition wire or spark plug cap (which also caused an engine misfire. It's a collapsing magnetic field.

There's often a combination of issues.

CAUSES:

Anything that causes AM radio interference or prevents an inductive tachometer fro reading steadily interference will likely cause sensor problems	This will cause interference all by itself.	Non-resister plugs and non-resister spark plug caps are often a problem.	Arcing spark plug wires  are always a BIG RF problem. When the arc occurs, the engine will miss.	Solid Core spark plug wires can cause problems - Sometimes you need to change to graphite core "suppression type" for dyno testing.	Older ignition systems and older CDI system replacement systems may produce loads of RF interference. If that's the problem, it's usually big and of you figure out how to solve it, email me. marc@factorypro.com	Tig / Mig / Arc welding in the building WILL cause huge dyno problems.	STRONG RF interference will  Blue Screen a computer.
	always causes a problem		always causes a problem		almost always causes a problem	always causes a problem	If the vehicle's ign system causes a Blue Screen and you have resister plugs, suppression spark plug wires and nobody is welding and no external high voltage arcing - you have a severe, maybe insurmountable issue. Maybe have to change ign system.

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Example:
CB350 vintage road race bike:

aftermarket CDI ignition system.
Non-resistor plugs. (radiative RF)
Solid core spark plug wires.

The RF induced spurious voltage spikes into the dyno's sensor wiring, evidenced by spiking rpm values on the dyno screen, preventing the dyno from stabilizing rpm or load.
RF interference levels can be so high that the computer might even "blue screen".

Resister plugs helped.
Running the dyno's load cell and speed sensor wires through steel conduit helped some (to prevent the dyno wiring from acting like an antenna).
Running resistor type spark plug wires would help.

In the end, the aftermarket CDI system in itself simply produced a lot of RF and it was just barely able to occasionally auto record data.